Ahmed Coulibaly in a video in which he says he carried out the attacks in the name of Islamic State

A Paris police insider may have aided Amedy Coulibaly's murderous rampage that culminated in the killing of four shoppers in a kosher supermarket in January.

Four people have been placed in custody in the investigation into the Paris terrorist attacks, which included the attack on satirical magazine Charlie Hebdo, according to judicial sources.

Among them is a relative of Coulibaly, the gunman who killed a policewoman in Paris' Montrouge and then four Jewish shoppers in a Hyper Cacher supermarket in Porte de Vincennes.

The man, named only by Amar R, was spotted near the kosher supermarket and even sighted in company of Coulibaly just before the 9 January attack, Europe 1 reported.

Amar R was followed by police hours before Coulibaly stormed the Hyper Cacher but arrested only on 23 January under a European arrest warrant issued by Spain for drug-trafficking and arms smuggling. He was never questioned by police about the Paris attack until now.

His partner, a policewoman named Emmanuelle, has also been put in custody. According to the report, the policewoman converted to Islam two years ago and was stationed at Rosny-sous-Bois, in the outskirts of Paris, where after the Paris attacks she allegedly consulted computer files on his partner.

The widow of Coulibaly, Hayat Boumeddiene, crossed into Syria on 8 January after travelling to Turkey on 2 January.

Reports that Coulibaly had accomplices emerged immediately after the attack.